1. Field of the Invention
This invention relates to a multi-layer extrusion die for use in simultaneously extruding thermoplastic resins of two or more types to obtain a multi-layer tubular resin.
2. Description of the Prior Art
A multi-layer tubular resin such as a multi-layer film is formed such that a plurality of resins different in performance from one another are combined to provide a product having a new performance, in which characteristics of the respective resins are efficiently used. Typically, the respective resins to be combined together have melting points and thermal resistances different from one another.
As shown in FIG. 1, the convertional multi-layer extrusion die used for forming the multi-layer film and the like as described above comprises an inner layer resin flow path 1 and an outer layer resin flow path 2 surrounding the inner layer resin flow path 1. Both an inner layer resin and an outer layer resin are heated from outside by a heater 4 provided on the outer periphery of a die body 3. For example, a resin having a high melting point is caused to flow through the outer layer resin flow path 2 disposed closer to the heater 4 and a resin having a low melting point is caused to flow through the inner layer resin flow path 1 disposed farther from the heater 4.
However, when the conventional multi-layer extrusion die as described above is used, such disadvantages have been presented that burning of resin occurs, a product having an unsatisfactory surface profile is obtained or non-formability is sometimes resulted because the inner layer resin having a low melting point is heated to more than a proper temperature. Further, in the case of forming a laminated film by use of a resin having a high melting point as an inner layer resin and a resin having a low melting point as an outer layer resin, the outer layer resin is overheated to a great extent in order to heat the inner layer resin, thus resulting in a by far larger problem.
To overcome this problem, there has been made a contrivance that the flow path for the resin having a low melting point (a resin being low in heat resistance) is made narrow and the retention time of the inner layer resin in the die is shortened. However, there have been restrictions due to the forming, thereby preventing obtaining of satisfactory results.
It has been proposed that a heat transfer medium for heating is circulated through an intermediate portion between the inner layer resin flow path and the outer layer resin flow path (Japanese Patent "Kokoku (Post-Exam. Publn.)" No. 5833/79 and Japanese Utility Model "Kokoku (Post-Exam. Publn.)" No. 22622/80), and that a heat insulating space is formed between a die for the inner layer and a die for the outer layer and these two dies are controlled in temperature independently of each other (Japanese Utility Model "Kokoku (Post-Exam. Publn.)" No. 4555/79).
However, with these multi-layer extrusion dies thus proposed, it is difficult to simplify the construction and the temperature control should not necessarily be easily effected.